What should I do??... in need of advice.?

I'm 19 and just started work at my summer job.
The job entails walking door to door fundraising for an environmental organization. At first I liked it and it came easily to me, I made way over my quota. the people I work with are accepting and cool even though we don't have very much in common.
After a few days though (where I was trained intensely...and had to read tons of training material) it was an extreme struggle just to earn a third of my quota. I don't really know what happened.
We work long hours from 1-10 at night.. and I find myself walking around these neighborhoods in the dark wishing I were nice and comfortable in my own home rather than cold outside in tears.
I was given a second chance by my manager.Should I still keep at it and hope that I improve? How could I improve?
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If you were doing good before, don't just stop believing in your abilities as a sales person just because your sales are down. Evaluate all the factors that contribute to the issue and go from there.

If there is something you can do better on the job based on what you perceive or based on constructive criticism by your manager, then fix it. If this is purely circumstantial and you can't help it, then why beat yourself up about it? Your going to have good days and bad days, good weeks and bad weeks.

If you don't like the job, look for something else. If you like the job, keep up a positive attitude and continue to act as confident as you were coming into it if at all possible.

The more flustered you get, the harder it's going to be to come across as genuine, layed back and friendly to your potential supporters/clients. That in itself will hurt your ability to be effective at this type of job.
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You said this is a summer job yet you are cold outside? This does not make sense. I suspect this is not a legit question.
If it is legit, my suggestion is that you find something that you enjoy doing. Sales or cold call solicitation is not for everyone. Anything involving approaching strangers and trying to convince them to do what you want requires a certain type of personality and if you do not have that, you are wasting your time. Life is too short to get locked into a job that you hate. If you can find something you like better, go there.